The present invention relates to semiconductor pressure sensors and, more particularly, to a semiconductor pressure sensor which is suitable for use in control of the ignition timing or fuel injection in automobiles and which enables high-precision temperature compensation over a wide range of temperatures.
In general, the semiconductor strain gage which constitutes a semiconductor pressure sensor is composed of a P-type diffused resistor formed on an N-type silicon substrate, and the temperature coefficient of resistance and the temperature coefficient of gage variation coefficient (resistance variation coefficient per unit of strain) are each dependent on the surface impurity concentration at the time of the diffusion.
It is disclosed in, for example, "Collection of Papers Scheduled to be Presented at 36th Meeting of Applied Physics Society 24a-D-6 (1975)", lines 16 to 19 on page 62 (presented by Shimazoe and Yamada), that, in the case where a pressure sensor in which a semiconductor strain gage of the type described above is formed on a silicon diaphragm in the form of a Wheatstone bridge, when this pressure sensor is driven with a constant current, the temperature coefficient of sensitivity reaches the minimum when the surface impurity concentration is 10.sup.18 atoms/cm.sup.3 and 10.sup.20 atoms/cm.sup.3, that is, the temperature variation coefficient of the semiconductor strain gage is zero at these surface impurity concentrations.
According to the statement in claim 1 of "Process for Producing Semiconductor Pressure-Sensitive Device", Japanese Patent Laid-Open No. 113379/1980 laid open to public inspection on Sept. 1, 1980, and the description in lines 3 to 6 on the right-hand column, page 353, it is possible to flatten the surface of the piezoresistance element used in a semiconductor pressure-sensitive device and minimize the temperature dependence of its sensitivity by setting the surface impurity concentration in the vicinity of 2.times.10.sup.20 atoms/cm.sup.3.
Pressure sensors, particularly those which are employed to control ignition timing or fuel injection in automobiles, are used in severe environments and are required to be capable of precisely measuring changes in the intake pressure within the intake manifold at from -40.degree. C. to 120.degree. C.
With the above-described prior art, however, although it is possible to eliminate the temperature dependence within a relatively narrow temperature range, more specifically from 10.degree. C. to 60.degree. C., the prior art is temperature-sensitive in low or high temperature regions other than the above-described temperature range, and it is necessary to provide temperature compensation in the high and low temperature regions. In such a case, since semiconductor strain gages have positive temperature characteristics in the low and high temperature regions (described later in detail), it was considered impossible to effect satisfactory temperature compensation in both the low and high temperature regions with thermistors having positive temperature characteristics only. Accordingly, when compensation over a wide range of temperatures is needed, the temperature compensating circuit is disadvantageously complicated.